The invention relates to a method for the production of an elongated superconductor, in strip form that has at least one conductor core. The superconductor is formed of a superconductor material with a metal-oxide high-Tc phase and is surrounded by a normally conductive material. In which method, a conductor initial product is produced from a powdery initial material of the superconductor material surrounded by the normally conductive material. The conductor initial product is then subjected to a plurality of forming steps which reduce the cross section, compress the initial material and contain at least two flattening steps, and to at least one annealing process. The invention also relates to a superconductor produced using this method. An appropriate production method and a superconductor produced using this method are disclosed, for example, in the publication xe2x80x9cPhysica Cxe2x80x9d, Vol. 250, 1995, pages 340 to 348.
Superconductive metal-oxide compounds having high critical temperatures Tc of more than 77 K are known, which are therefore also referred to as high-Tc superconductor materials or HTS materials and, in particular, allow a liquid-nitrogen (LN2) cooling technique to be used. Such metal-oxide compounds include, in particular, cuprates of specific material systems, in particular such as the base type Yxe2x80x94Baxe2x80x94Cuxe2x80x94O which contains rare earths, or the base type Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O or (Bi, Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O, which do not contain any rare earths. A number of superconductive high-Tc phases may occur within individual material systems such as the bi-cuprates, which differ in the number of copper-oxygen network levels or layers within the crystalline unit cells, and in the various critical temperatures Tc.
Attempts have been made to use the known HTS materials to produce elongated superconductors in wire or strip form. One method that is regarded as being suitable for this purpose is the so-called xe2x80x9cpowder-in-tube techniquexe2x80x9d, which is known in principle from the production of superconductors using the classic metallic superconductor material Nb3Sn. In this technique, a generally powdery initial material which is composed of the HTS material but in general does not yet possess the desired superconductive high-Tc phase, or possesses it only to a minor extent, is introduced into a tubular sheath or into a matrix composed of a normally conductive material, in particular composed of Ag or an Ag alloy, in order to produce conductors composed of HTS material as well. The conductor initial product which be obtained in this way is then changed to the desired final dimension by forming processes which, if required, can be interrupted by at least one heat treatment process at an increased temperature. After this, the conductor intermediate product obtained in this way is subjected to at least one annealing process in order to set or optimize its superconductive characteristics and to form the desired high-Tc phase, which annealing process may be interrupted, if required, by a further forming step.
If appropriate high-Tc superconductors or their conductor initial products or their conductor intermediate products are grouped in a manner known per se, then it is also possible to obtain conductors having a plurality of superconductive conductor cores, so-called multicore or multifilament superconductors.
The known single-core or multicore superconductors using the HTS material are preferably in strip form. In order to obtain a corresponding conductor end product with this form, the literature reference mentioned initially states that a rolling process must be provided. However, before this rolling process, a generally cylindrical, preformed and precompressed composite body must be produced from the conductor initial product, whose conductor cores are generally disposed distributed uniformly, when seen over the cross section. This composite body, which is referred to in the following text as a raw conductor, is then changed to the flat strip form by a rolling process. The rolling process includes a plurality of rolling steps, in order in this way to achieve a texture required for a high current carrying capacity, that is to say with the crystal layers of the superconductive phase being aligned largely parallel. To do this, the initial material of the superconductor must be compressed as much as possible by the rolling process during the forming of the raw conductor. However, it has been found that, beyond a certain powder density, further forming results in inhomogeneities such as the so-called xe2x80x9csausagingxe2x80x9d, that is to say the conductor core being constricted or reduced along the length of the conductor, or cracks occurring. This limits the amount of compression from the rolling process.
For this reason, the method that is disclosed in the literature reference mentioned initially allows compression to be carried out only until corresponding inhomogeneities occur. Thus, as a rule, the rolling process is carried out so as to ensure that, after the first rolling step, the next rolling step results in an at least largely equivalent reduction in the thickness of the individual conductor core, or percentage thickness reduction. The term thickness reduction in this case relates to the parameter (dixe2x88x92di+1)/di, where the index i=0, 1, 2 . . . refers to the number of the respective rolling step, and di refers to the thickness of the respective conductor core after the i-th rolling step. d0 is the initial thickness, before rolling.
In the method for production of an HTS strip conductor that is disclosed in Published, European Patent Application EP 0 435 286 A, the rolling process is carried out such that the thickness reduction in a second rolling step which follows the first rolling step is less than that from the preceding step.
It is accordingly an object of the invention to provide a method for producing a superconductor, in strip form, having a high-tc superconductor material that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which inhomogeneity problems are reduced, thus resulting in a superconductor, in strip form, having an increased current carrying capacity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a production method, which includes:
producing a conductor initial product having a plurality of conductor cores formed of a powdery initial material being a superconductor material and surrounded by a normally conductive material;
subjecting the conductor initial product to a first flattening step such that a percentage reduction in a thickness of a cross section of at least one conductor core is between 3 and 10% and powder fluidity is maintained in the at least one conductor core;
subsequently subjecting the conductor initial product to a second flattening step carried out such that the percentage reduction in the thickness of the cross section of the at least one conductor core is at least 5% greater than that from the first flattening step; and
annealing the conductor initial product resulting in an elongated superconductor having the plurality of conductor cores composed of the superconductor material with a metal-oxide high-Tc phase and being in a strip form.
The object is achieved according to the invention in that a flattening step which follows a preceding flattening step can be carried out in such a way that the percentage reduction in the thickness of the cross section of the at least one conductor core resulting from the subsequent flattening step is greater than that from the preceding flattening step.
The measures according to the invention thus result in progressive compression of the powdery initial material. That is to say the percentage thickness reduction of the at least one conductor core increases during the raw conductor forming process. In this case, the term raw conductor refers to the composite body before the first flattening step, in particular by rolling. The procedure is based on the fact that the powder fluidity during the raw conductor forming process must be maintained for as long as possible, in order in this way to achieve optimum compression of the powder. Specifically, if all the rolling steps produce a constantly low forming level of, for example, 10%, then the powdery initial material will reach a virtually solidified state after a small number of rolling steps. Therefore, the powder particles can no longer move owing to the friction between them, since the externally applied force is no longer sufficient to overcome the friction forces. This prevents the powder from flowing; the powder particles are now only pressed against one another, as a result of which cavities remain, and the compression process stagnates. If the amount of forming is constantly high, the powder will be compressed so severely just in the first step that inhomogeneities such as cracks or fractures will occur if further forming is carried out. This results in that the powder is then no longer fluid. Degressive compression as is intended to be provided, for example, in the cited Published, European Patent Application EP 0 435 286 A, and in which only lesser forming levels follow a high forming level results in that the compression process resulting from powder fluidity comes to end even after the first rolling step.
In contrast, the measures according to the invention achieve progressive compression, or forming. Increasing the forming force thus makes it possible to overcome the previously blocked state of the powder, due to friction, which can lead to further fluidity and compression. The powder fluidity can thus be maintained for as long as possible as the forming level increases and the density rises. This prevents high partial compression processes and microscopic fractures, which lead to the above-mentioned inhomogeneities, before the conductor end product reaches the desired density.
The advantages achieved by the measures according to the invention are thus that deliberate selection of the compression parameters results in that inhomogeneities cannot occur until considerably high powder densities thus, by virtue of the increased homogeneous powder densities, making it possible to ensure a correspondingly greater current carrying capacity.
The method according to the invention is used particularly advantageously for production of a single-core superconductor, or preferably a multicore superconductor, whose at least one conductor core is surrounded by a normally conductive material which contains Ag or is composed of Ag. This makes it easier to form the superconductive phase in an atmosphere containing oxygen.
In accordance with an added feature of the invention, there is the step of subjecting the elongated superconductor having the strip form to at least one more further flattening step.
In accordance with an additional feature of the invention, there is the step of performing the first flattening step and the second flattening step as rolling steps.
In accordance with another feature of the invention, there is the step of carrying out the rolling steps with pairs of rollers having differing roller diameters.
In accordance with another added feature of the invention, there is the step of performing the second flattening step such that the percentage reduction in the thickness of the cross section is at least 20% greater than that from the first flattening step.
In accordance with another additional feature of the invention, there is the step of performing the at least one more further flattening step such that the percentage reduction in the thickness of the cross section is at least 20% greater than that from the first flattening step.
In accordance with yet another feature of the invention, there is the step of producing the conductor initial product using a powder-in-tube technique and further processing the conductor initial product with at least one forming step which reduces the cross section, to form a raw conductor which is subsequently subjected to the first flattening step.
In accordance with a further added feature of the invention, there is the step of providing a material containing at least Ag as the normally conductive material.
In accordance with a further additional feature of the invention, there is the step of forming the plurality of conductor cores formed with the high-Tc phase to be composed of a superconductive bi-cuprate.
In accordance with a concomitant feature of the invention, there is the step of subjecting the elongated superconductor to at least one further annealing process step.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing a superconductor, in strip form, having a high-tc superconductor material, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.